Moving coil loudspeaker using thin magnet

ABSTRACT

A field assembly for a loudspeaker having a base plate, a front plate and a permanent magnet sandwiched therebetween, a cylindrical post being secured to the center of the base plate in such a manner as to leave a gap for the movements of a voice coil therein. The base plate is provided with an embossment creating additional space for voice coil travel. Instead of an embossment integral with the base plate, a separate disc member with a peripheral flange may be used.

United States Patent Derkacz MOVING COIL LOUDSPEAKER USING THIN MAGNET John J. Derkacz, Chicago, Ill. Assignee: Alpha Products, Inc., Chicago, 111. Filed: Aug. 27, 1970 Appl. No.: 67,517

Inventor:

US. Cl. ..l79/ll5.5 R, 179/117, 179/119 R, 335/231 Int. Cl ..H04r 9/02 Field ofSearch ..179/l 15.5 R, 115.5 RC, 115.5 PC, 179/117 R, 119 R, 120; 335/231; 181/31 References Cited UNITED STATES PATENTS Zuerker ..179/l15.5 R Engholm ..l79/115.5 R

[451 July 25, 1972 2,862,069 11/1958 Marchand et al. ..179/l15.5 R

FOREIGN PATENTS OR APPLICATIONS 408,284 4/1934 Great Britain 179/1 15.5 R

491,509 9/1938 Great Britain ..179/l15.5 R

Primary Examiner-Kathleen H. Clafiy Assistant Examiner-Thomas L. Kundert Attorney-Silverman & Cass [5 7] ABSTRACT A field assembly for a loudspeaker having a base plate, a front plate and a permanent magnet sandwiched therebetween, a cylindrical post being secured to the center of the base plate in such a manner as to leave a gap for the movements of a voice coil therein. The base plate is provided with an embossment creating additional space for voice coil travel. Instead of an embossment integral with the base plate, a separate disc member with a peripheral flange may be used.

4 Claims, 3 Drawing figures I aim MOVING COIL LOUDSPEAKER USING THIN MAGNET BACKGROUND OF THE INVENTION l. Field of the Invention The invention relates generally to audio reproducing devices and more particularly is concerned with improvements in a loudspeaker of the permanent magnet variety which utilizes a relatively flat annular magnet.

In the permanent magnet type of loudspeaker a strong magnetic field is established in an air gap in a magnetic circuit which includes a permanent magnet. The magnetic circuit usually includes a housing or pot, as it is known in the trade, and a powerful permanent magnet, the combination in most cases being symmetrical about a single axis to provide uniform distribution of the flux. The magnet housing is rigidly secured to a metal basket which mounts a paper cone to which the voice coil is cemented. The voice coil is disposed at the apex of the cone and is cylindrical in configuration to enable it to extend into the cylindrical air gap between the pole pieces of the magnetic circuit.

2. Description of the Prior Art and Departures therefrom The technical details of loudspeaker operation are fairly well known, and several structures of the type indicated above are illustrated and described in U.S. Pat. Nos. 3,133,162; 3,176,086 and 3,453,400. Reference may be had to these patents for structural details of loudspeakers of a variety in which the magnet is in the center of the magnetic circuit as a core, while the looping of return portions of the magnetic circuit are on the exterior of the magnet.

Recently loudspeakers have been constructed in which the magnet is in effect on the outside of the circuit and the return portions are on the inside, still retaining the basic symmetrical and coaxial arrangement. Specifically, flat annular toroidal magnets have been manufactured, of hardened mixtures of magnetic particles and ceramics, these being referred to in the I trade as ceramic magnets. When mounted in suitable metal field pieces, these ceramic magnets make up into motors," as the assemblies are sometimes called, which have very small axial lengths. Thus the loudspeaker itself has very short axial dimensions, enabling the manufacture of compact radios and television receivers.

A typical motor for a loudspeaker using a ceramic magnet as heretofore known utilized a structure requiring three metal pieces in addition to the magnet. There was a base plate in the form of an annular washer with a center perforation, having a cylindrical center post staked or force-fitted therein. The ceramic magnet, which is annular in configuration was arranged coaxial of the post, engaged against the top of the base plate. A flat washer was disposed overlying the magnet and having a central opening coaxial with the post but of a diameter larger than that of the post. The post extended axially at least to the plane of the outer face of the washer, usually slightly beyond, so that the air gap was formed as a cylindrical space between the post and the inner surface of the perforation in the washer.

The base and washer were formed of cold rolled steel by any suitable method such as stamping or machining. The ceramic magnet was quite fiat compared to the cylindrical or core type of permanent magnet heretofore used in the loudspeakers.

It is a well known fact that the ceramic magnet is the most expensive part of the motor assembly. There has, therefore, been a long-felt desire in industry to find a way to reduce the high cost of the magnet. To this end, it was suggested to reduce the width of the magnet. However, this suggestion gave rise to another problem, namely that of shortening the range over which the voice coil could travel caused by the magnet having a smaller width. As is well known, in this type of magnetic field structure, the voice coil is moving forth and back in the axial direction of the motor assembly, and the magnet is one of the elements contributing substantially to the dimension of the overall width of the motor assembly.

To overcome the problem of diminished range of voice coil travel, the invention envisions a structure wherein the base plate is provided with an embossment in its center portion to which the cylindrical post is secured. The embossment provides additional space on the inner side of the base plate which space permits a greater range of travel for the voice coil.

The prior art does not teach the concept of a loudspeaker assembly having a thin magnet compensated by an embossment in the base plate.

Accordingly it is an object of this invention to provide a motor for a loudspeaker, comprising a flat permanent magnet disposed between a base plate and a front plate wherein the base plate is provided with an embossment for enlarging the range of voice coil travel.

It is a further object of the invention to provide a loudspeaker field assembly, wherein the permanent magnet is of a small width relative to the width of the base and front plates without the range of voice coil travel being diminished thereby.

Still a further object of the invention is to provide a loudspeaker field assembly, in which the embossment is an integral portion of the base plate.

Another object of the invention is to provide a loudspeaker assembly, wherein the embossment feature of the base plate is obtained by a part separate from the base plate, but fastened thereto.

Other objects and advantages not specifically enumerated will become apparent to those skilled in this art as a description of the preferred embodiments of the invention is set forth in detail. The drawings which accompany this specification illustrate both embodiments of the structure of the invention.

SUMMARY OF THE INVENTION A loudspeaker field structure comprising a flat permanent magnet having a small width. A base plate is provided with an embossment in the center portion such as to form additional space for voice coil travel thereby compensating for space lost through the small width of the permanent magnet.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a sectional view through a loudspeaker motor assembly of the type which uses a Hat annular magnet and a base plate constructed in accordance with the invention;

FIG. 2 is a plan view of the motor assembly;

FIG. 3 is an exploded view illustrating the base plate with an embossment, and the cylindrical post;

FIG. 4 is a fragmentary sectional view of another embodiment of the invention illustrating a base plate having a disc with a flange.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, a loudspeaker assembly is illustrated in sectional view at 10 in FIG. I. A loudspeaker motor 11 is secured to the bottom end of a metal basket 12 by some appropriate means with which the invention is not concerned. The motor may be encased in a cap or housing which, for the same reasons, is not shown. The speaker cone 13 is secured at its apex to a cylindrical form 14 upon which the voice coil 15 is wound. The voice coil is coaxial with and extends into a cylindrical air gap 16 formed in the magnetic circuit of the motor 11. The cone and cap are usually made of paper as is the voice coil form 14. The motor 11 comprises a base plate 17, a permanent magnet 18 and a front plate 19 secured to one another in this order by appropriate means. The base plate 17 and the front plate are of about the same width.

The novelty of the invention is primarily in the construction of the base plate 17.

As previously discussed, the use of a thin permanent magnet would reduce the range of voice coil travel. In order to avoid such reduction, an embossment 20 is provided in the central portion of the plate 17. This is done by subjecting the center portion of the base plate to a pressing operation whereby the portion 20 is drawn out and away from the plane of the base plate 17. This operation produces an annular space 21 as shown in FIGS. 1 and 3, which provides an extension to the air gap 16 in axial direction. With such extension the voice coil is capable of moving in the air gap further than it could do in the case of a flat base plate, i.e., without an embossment 20in the plate.

A cylindrical post 22 having a reduced end portion 23 is secured in a suitable manner to the aperture 24 of base plate 17. The previously noted cylindrical form 14 supporting the voice coil 15 is mounted by appropriate means on the upper end of the cylindrical post 22 so that the coil is capable of sliding thereon.

It will be appreciated that the feature of providing the base plate with an embossment affords, also, effective means for adjusting the motor characteristics to conditions as desired. Expressed otherwise, where longer voice coil travel is required the embossment will be made of greater height such as by a deeper drawing operation of the center portion of the base plate.

Another mode of achieving the object of adjusting the motor. characteristics to conditions as desired, or in other words, adapting the motor assembly so as to obtain a greater range of voice coil travel, is illustrated in FIG. 4.

In this embodiment the cylindrical post 22 is connected to a disc member 35 by suitable means which is of no concern to this application. The disc member is fastened to the base plate in some conventional way. The disc member may be of annular configuration and is provided with an annular depending flange 36. The flange 36 may be of smaller or larger height, the height being, as is readily recognized, a function of the range the voice coil is required to travel, that is, the greater the range of voice coil travel the greater will be the height selected for flange 36.

Other constructional details should be obvious from the drawings and, likewise, it is obvious that variations of a wide range may be made without departing from the spirit or scope of the invention as defined by the appended claims.

What is desired to secure by Letters Patent of the United States is:

lclaim:

l. A motor for a loudspeaker which comprises a. a base plate formed of an integral flat member having a dished portion in the center thereof upset by embossing to produce a flat annular flange in one plane and the dished portion comprising a flat part in a parallel plane spaced therefrom and connected by an annular imperforate web, said dished portion having an aperture in the center thereof,

b. a cylindrical post, the lower end thereof being fastened to the base plate in said aperture and having an end circumferential surface of predetermined outside diameter,

c. an annular magnet having an aperture in the center thereof mounted on the flat annular flange of the base plate coaxially with the post,

. an annular front plate having a central aperture and being attached to the magnet such that the central aperture is coaxial with said post, said central aperture having a predetermined inside diameter larger than the said outside diameter of the cylindrical post such as to form an air gap therewith, the magnet being sandwiched between the base plate and the front plate with its aperture adjacent to the end circumferential surface of the post, the said air gap being adapted to receive therein a single voice coil of helical configuration having an inner diameter slightly larger than the outside diameter of the post and an outside diameter slightly smaller than the diameters of the apertures of the magnet and the front plate so as to be capable of sliding along the post,

f. and wherein said embossed dished portion of the base plate is adapted to afford additional space for voice coil travel. 2. A motor for a loudspeaker as set forth in claim 1, wherein the magnet is of a small width relative to the width of the base plate.

3. A motor for a loudspeaker as set forth in claim 2, wherein the embossment has a diameter larger than the outside diameter of the voice coil.

4. A motor for a loudspeaker as set forth in claim 3, wherein the height of the embossment is at least equal to the difference between the width of the magnet and the width of the base plate. 

1. A motor for a loudspeaker which comprises a. a base plate formed of an integral flat member having a dished portion in the center thereof upset by embossing to produce a flat annular flange in one plane and the dished portion comprising a flat part in a parallel plane spaced therefrom and connected by an annular imperforate web, said dished portion having an aperture in the center thereof, b. a cylindrical post, the lower end thereof being fastened to the base plate in said aperture and having an end circumferential surface of predetermined outside diameter, c. an annular magnet having an aperture in the center thereof mounted on the flat annular flange of the base plate coaxially with the post, d. an annular front plate having a central aperture and being attached to the magnet such that the Central aperture is coaxial with said post, said central aperture having a predetermined inside diameter larger than the said outside diameter of the cylindrical post such as to form an air gap therewith, e. the magnet being sandwiched between the base plate and the front plate with its aperture adjacent to the end circumferential surface of the post, the said air gap being adapted to receive therein a single voice coil of helical configuration having an inner diameter slightly larger than the outside diameter of the post and an outside diameter slightly smaller than the diameters of the apertures of the magnet and the front plate so as to be capable of sliding along the post, f. and wherein said embossed dished portion of the base plate is adapted to afford additional space for voice coil travel.
 2. A motor for a loudspeaker as set forth in claim 1, wherein the magnet is of a small width relative to the width of the base plate.
 3. A motor for a loudspeaker as set forth in claim 2, wherein the embossment has a diameter larger than the outside diameter of the voice coil.
 4. A motor for a loudspeaker as set forth in claim 3, wherein the height of the embossment is at least equal to the difference between the width of the magnet and the width of the base plate. 